A ladle is a vessel that is used to hold or transport a batch of liquid metal during metallurgical operations. The ladle lining includes a generally horizontal ladle bottom that is fashioned from refractory materials, such as blocks, bricks, or castable materials. The ladle bottom includes an outlet defining a bore that provides a generally vertical conduit for the outflow of liquid metal during casting. Typically, the outlet is encircled by or encompassed within a block called the nozzle block. The nozzle block is set within and is surrounded by the remainder of the ladle bottom, which most commonly comprises bricks. To facilitate the casting process, the nozzle block is very typically offset from the center of the ladle bottom and is generally located closer to the periphery of the ladle bottom than to the center of the ladle bottom.
A layer of slag frequently covers the top surface of the liquid metal, such as in the production of steel. During casting, the liquid metal is drained from the ladle though the outlet located in the ladle bottom. While draining, the metal will, desirably and advantageously, completely empty from the ladle without contamination of the metal by slag or other contaminates, such as entrained particles. Contamination is undesirable and may cause difficulties in casting or refining operations as well as defects in the intermediate or final metal products.
Contamination can occur from both floating and entrained slag. Slag is typically less dense than liquid metal and generally floats in a separated layer on the surface of a quiescent batch of liquid metal. During the draining of the liquid metal, slag can become entrained within the flowing stream. Entrainment is the presence of slag particles in the molten steel. Entrainment often occurs when turbulent flow of the liquid metal disturbs the interface between molten metal and slag. Such turbulence can cause molten metal and slag to mix. Under quiescent conditions, entrained slag would eventually float to the surface; however, the turbulence of casting can maintain a substantial amount of entrained slag in the molten metal. Ideally, any solution to the problem of slag contamination would address both floating and entrained slag.
As the metal drains from the ladle, the floating slag approaches the outlet and the likelihood of contamination of the metal stream by slag increases. An operator will stop the pour when he detects slag in the molten metal stream exiting the ladle. The operator may even stop the pour prematurely to avoid slag in the ladle outflow. The slag and metal remaining in the ladle are discarded. Discarding metal decreases yield, which is costly and inefficient but, at the same time, is necessary to reduce slag contamination.
Various methods and articles exist to detect slag in the ladle or the ladle outflow. Frequently, these methods require action by the operator and include electronic, electro-magnetic and sonic detection devices that are placed inside and outside the ladle. For example, a detector placed in the ladle can detect a drop in the level of molten metal by measuring a change in detector output when floating slag intersects the detection region. Similarly, sonic pulses can identify the presence of slag in the ladle outflow. Both techniques only detect the presence of slag and do not actively decrease the presence of slag in the outflow.
Prior art includes articles designed to reduce the outflow of slag from the ladle. U.S. Pat. Nos. 4,746,102 and 5,879,616 teach ladle bottoms having a small well immediately above the ladle outlet. Both patents describe the well as preferentially collecting molten metal instead of slag, thereby improving yield as the ladle empties. Unfortunately, the patents only prevent floating slag from exiting the ladle. Entrained slag is free to exit the ladle.
U.S. Pat. No. 5,196,051 describes a ladle bottom for reducing entrained slag. The ladle bottom comprises means for entrapping slag before the slag reaches the ladle outlet. The means extend upwards from the ladle bottom and include elongated castellations that approach the outlet. One embodiment shows castellations radiating symmetrically from the outlet. The symmetrical castellations are described as reducing vortexing, which leads to slag entrainment. Notably, the castellations are not described as promoting a reduction of entrained slag already present in the molten metal.
Prior art does not teach a ladle bottom that simultaneously reduces the outflow of both entrained slag and floating slag. A need remains for an article capable of capturing entrained slag and allowing molten metal to flow from a ladle before floating slag. Preferably, the article could be quickly installed in an existing ladle bottom.